Liquid crystal displays, such as liquid crystal TV monitors and video camcorder monitors, have recently been required to operate at low voltages with low power consumption. To meet such demands, application of optically anisotropic elements using a liquid crystal substance has been studied to increase the utility of a light source or to improve viewing angle characteristics of liquid crystal displays. That is, it has been studied to make use of the aligning properties and anisotropic physical properties of a liquid crystal substance, such as refractive index, dielectric constant, and magnetic susceptibility, in the form of an optically anisotropic element, such as a retardation film, a polarizer, a polarizing prism, or a reflective film.
Such an optically anisotropic element is obtained by causing a liquid crystal compound having a polymerizable moiety or a polymerizable composition containing a liquid crystal compound having a polymerizable moiety to polymerize by irradiation with energy rays, such as ultraviolet rays, while maintaining the compound or the composition in an aligned phase. In other words, the resulting optically anisotropic element has the liquid crystal molecules fixed in their aligned state and is designed to exhibit optical anisotropy based on highly structurally controlled optically active moiety thereof.
In particular, addition of an optically active compound to a liquid crystal composition induces the liquid crystal molecules to take helical molecular alignment and thus allows for the manipulation of the optical properties of the resulting optically anisotropic element.
The helical pitch of the helical structure depends on the helical twisting power inherent to an optically active compound and the amount of the compound added. The lower the helical twisting power, the longer the helical pitch. A larger amount of an optically active compound is needed to obtain a shorter helical pitch. However, as the amount of an optically active compound added increases, the performance properties as a liquid crystal material generally reduce, which can result in various problems, such as increase in viscosity, decrease in response time, increase in driving voltage, narrowing of temperature range for liquid crystal phase, and drop of isotropic phase transition temperature. Therefore, an optically active compound with larger helical twisting power has been demanded.
However, none of the optically active compounds hitherto reported is satisfactory (see, e.g. Patent Document 1 to 7).
Patent Document 1: JP 2-305888A
Patent Document 2: JP 3-72445A
Patent Document 3: JP 4-317025A
Patent Document 4: JP 9-506088A
Patent Document 5: JP 2003-55661A
Patent Document 6: JP 2003-137887A
Patent Document 7: JP 2005-281223A